Myeloperoxidase (MPO) is a heme-containing enzyme found in neutrophilic granulocytes (neutrophils) and monocytes. MPO is one member of a diverse protein family of mammalian peroxidases that also includes eosinophil peroxidase (EPO), thyroid peroxidase (TPO), salivary peroxidase (SPS), lactoperoxidase (LPO), prostaglandin H synthase (PGHS), and others. The mature enzyme is a dimer of identical halves. Each half molecule contains a covalently bound heme that exhibits unusual spectral properties responsible for the characteristic green color of MPO. Cleavage of the disulphide bridge linking the two halves of MPO yields the hemi-enzyme that exhibits spectral and catalytic properties indistinguishable from those of the intact enzyme. The enzyme is activated by hydrogen peroxide, the source of which can be superoxide dismuatase (SOD)-catalyzed NADPH-derived superoxide anion and xanthine oxidase-derived superoxide anion and hydrogen peroxide formed upon purine oxidation. The main physiological substrates of MPO are halides (e.g. chloride) and pseudohalides (like thiocyanate), forming microbicidal hypohalous acids like hypochlorous acid (bleach) and hypothiocyanous acid (J. Clin. Biochem. Nutr., 2011, 48, 8-19).
Neutrophils play an important microbicidal role by phagocytosing (engulfing) and killing microorganisms. The engulfed load is incorporated into vacuoles, termed phagosomes, which fuse with granules containing myeloperoxidase to form phagolysosomes. In phagolysosomes the enzymatic activity of the myeloperoxidase leads to the formation of hypochlorous acid, a potent bactericidal compound (Free Radical Biology & Medicine, 2010, 49, 1834-1845). Hypochlorous acid is oxidizing in itself, and reacts most avidly with thiols and thioethers, but also converts amines into chloramines, and chlorinates aromatic amino acids.
MPO can also be released to the outside of the cells where the reaction with chloride can induce damage to adjacent tissue. In addition to this controlled release of MPO, neutrophils can also cast webs of decondensed DNA interspersed with intracellular proteins such as MPO into the extracellular space, so called neutrophil extracellular traps (NET). These NETs are thought to play a role in host defense towards extracellular microbes and has also been suggested to be an important pathophysiological mechanism in acute inflammatory diseases such as the immunothrombosis occurring in sepsis (Nature Rev. Immunol, 2013, 13, 34) and in autoimmunity, such as in systemic lupus erythematosus (SLE) (J. Immunol., 2011, 187, 538-552). Notably, MPO is required for NET formation (Blood, 2011, 117, 953).
Systemic levels of MPO is a well-described risk factor for various cardiovascular diseases (e.g. heart failure, acute coronary syndrome, myocardial infarction, stable coronary artery disease and atherosclerosis related conditions (Circulation, 2003, 108, 1440-1445; N. Engl. J. Med., 2003, 349, 1595-604; J. Am. Coll. Cardiol., 2007, 49, 2364-70). The role of MPO in these morbidities is not only related to the oxidative damage caused by the enzymatic products, but also a due to a consumption of nitric oxide, an important regulator of vascular and cardiomyocyte relaxation. Importantly, the contribution of MPO to cardiovascular diseases is not only via infiltration of neutrophils and monocytes into the vasculature and the myocardium, but also via the strategic deposition of extracellular MPO on proteoglycans on the basolateral side of the endothelium (Science, 2002, 296, 2391).
A causative role of MPO for the development of cardiovascular disease is also supported by the lower cardiovascular morbidity in MPO-defective subjects (Acta Haematol., 2000, 104, 10-15) and reduced coronary flow reserve (J. Biomed Sci., 2004, 11, 59-64) and increased mortality in subjects carrying a gain-of-function mutation in the MPO-promoter (New Engl. J. Med, 2004, 350, 517; Free Rad. Biol. & Med, 2009, 47, 1584; J. Biol. Chem., 1996, 271, 14412-14420). A direct effect on vascular flow and relaxation was also observed after administration of MPO in pigs (Eur. Heart J., 2011, 33, 1625).
Linkage of MPO activity to diseases has thus been implicated in cardiovascular diseases with microvascular inflammation and reactive fibrosis including heart failure such as heart failure with preserved ejection fraction, heart failure with reduced ejection fraction, acute coronary syndrome, myocardial infarction, stable coronary artery disease and atherosclerosis related conditions.
Although considerable progress has been made in the understanding and treatment of heart failure (HF), morbidity and mortality due to HF remain high. The main cause of cardiac remodeling, which underlies HF development, is increased ventricular wall stress as a result of sustained hypertension, myocardial infarction, valvular insufficiency or other events. Cardiac remodeling is referred to as any change in cardiac structure, dimension, mass or function and, although it is initially a compensatory mechanism to maintain cardiac output it may result in decompensation and HF development (J. Am. Coll. Cardiol., 2000, 35, 569-582). Main processes that contribute to cardiac remodeling are cardiomyocyte hypertrophy (growth), fibrosis and inflammation (Nat. Rev. Mol. Cell. Biol., 2006, 7, 589-600; Circ. Res., 2010, 106, 47-57).
Currently, the cornerstone of HF treatment is based on reduction of ventricular wall stress and it consists mainly on the use of inhibitors of the renin-angiotensin-aldosterone systems, such as angiotensin-converting enzyme-inhibitors (ACE-I), of β-adrenergic blockers and of diuretics (Eur. Heart J., 2012, 14, 803-869). Despite treatment, HF mortality is still high and about 50% of all patients die within 5 years after first diagnosis (J. Am. Coll. Cardiol., 1999, 33, 734-742). New treatment options directly focused at the major molecular and cellular processes driving cardiac remodeling are therefore urgently needed.Heart failure can be sub-divided in HF with reduced ejection fraction (HFrEF) and with preserved ejection fraction (HFpEF) (Curr. Heart Fail. Rep., 2012, 9, 363-368). HFrEF and HFpEF differ with regard to pathophysiology, clinical characteristics and treatment. HFrEF is often referred to as systolic HF and treatment with ACE inhibitors, β-blockers and diuretics have successfully reduced mortality and morbidity rates in patients with HFrEF. In contrast, for HFpEF, which is often denoted as diastolic HF, no treatment has to date convincingly shown to reduce mortality or morbidity. This is alarming, since the incidence and prevalence of HFpEF is rising, both in absolute terms and relative to HFrEF (Eur. Heart J., 2013, 34, 1424-1431). A key characteristic of HFpEF is reduced contractility and relaxation of the ventricular wall. Cardiac fibrosis is an important contributor of this stiffening of the ventricular wall. Targeting cardiac fibrosis is therefore an potential therapeutic strategy for HFpEF patients, but also in HFrEF patients fibrosis plays an important role. Moreover, HFpEF may transit into cardiac dilatation and into HFrEF. It will therefore be important to investigate whether pharmacological targeting of cardiac fibrosis could halt or attenuate HFpEF and its potential transition into HFrEF. Microvascular inflammation, resulting in interstitial fibrosis appears to play an important role in HFpEF development (J. Am. Coll. Cardiol., 2013, 62, 263-271) and an association has been demonstrated between HF and the inflammatory enzyme, myeloperoxidase (MPO) (J. Am. Coll. Cardiol., 2007, 49, 2364-2370). In terms of fibrosis, there are data suggesting that the MPO-product hypochlorous acid (HOCl), is an important regulatory switch modulating extracellular matrix proteins, including metalloproteinases (MMPs) (J. Biol. Chem., 2001, 276, 41279-41287; J. Biol. Chem., 2003, 278, 28403-28409). This is also supported by the attenuation of angiotensin II-induced atrial fibrosis and reduced MMP-activity observed in MPO-deficient. Moreover, addition of recombinant MPO to Mpo−/− mice resulted in atrial fibrosis indicating that increased MPO activity is sufficient for induction of fibrosis (Nat. Med., 2010, 16, 470-474). In a post-myocardial infarct (MI) model Mpo−/− mice also showed diminished ventricular remodeling and improved function (J. Exp. Med., 2003, 197, 615-624). Together these results indicate that MPO activity is a key player for structural remodeling of the myocardium under pathological conditions.
Pharmacological inhibition of MPO activity could attenuate cardiac fibrosis and preserve cardiac function under conditions that may trigger (diastolic) HFpEF and (systolic) HFrEF. In particular stiffening of the heart as a result of extensive fibrosis could be prevented and may preserve cardiac function.
There is a need for an orally active inhibitor of MPO for the treatment of e.g. heart failure and coronary artery disease related conditions. In order to increase the therapeutic index of such a medication, it is necessary to obtain an MPO inhibitor being selective for MPO over other peroxidases such as for instance TPO to reduce the risk of thyroid related adverse events. It is considered that a high selectivity for MPO over TPO may reduce the risk of a growth of the thyroid gland (Pharm. Res., 2013, 30, 1513-24. Furthermore, it is desirable that an MPO inhibitor for the use in cardiovascular therapy would have limited blood brain barrier penetrating properties as to minimize its effect in the central nervous system (CNS).
WO2003/089430, WO2005/037835, WO2007/120097, WO2007/120098 and WO2007/142576 disclose thioxantine derivatives and the use thereof as MPO inhibitors in therapy.
WO2006/062465 and WO2007/142577 disclose 2-thioxo-1,2,3,4-tetrahydro-pyrrolo[3,2-d]pyrimidin-4-one derivatives claimed to be inhibitors of MPO. It is stated that the compounds may show selectivity against related enzymes such as TPO.
WO2009/025618 discloses thioxantine and 2-thioxo-1,2,3,4-tetrahydro-pyrrolo[3,2-d]pyrimidin-4-one derivatives and the use of MPO inhibitors for the treatment of multiple system atrophy (MSA) and Huntington's disease (HD) and for neuroprotection.
J. Labelled Compounds and Radiopharmaceuticals. 2012, 55, 393-399, discloses some tritiated, 13C and 14C labeled thioxantine derivatives as well as a 14C labeled pyrrolo[3,2-d]pyrimidin-4-one compound. The compounds are stated to be inactivators of MPO.
J. Biol. Chem., 2011, 286, 37578-37589, discloses certain thioxantine derivatives. The compounds are stated to inhibit MPO in plasma and decrease protein chlorination in a mouse model. The compounds are also claimed to be poor inhibitors of TPO.
WO2013/068875 discloses thiopyrimidone derivatives claimed to be MPO inhibitors.
An object is to provide novel MPO inhibitors useful in therapy. A further object is to provide novel compounds having improved selectivity for the MPO enzyme over the TPO enzyme and/or having limited blood brain barrier penetrating properties.